This invention relates to mass spectrometry and, particularly, to a conduit for conducting ions from a high-pressure ion source to a mass analyzer in mass spectrometry apparatus.
In mass spectrometry apparatus, an interface must be provided between a source of ions to be analyzed, which is typically at high-pressure (typically about atmospheric pressure), and the enclosure for the mass analyzer, which is typically at very low pressure. In one approach, a tube, having a bore usually of capillary dimension, serves as a conduit for the ions. The capillary is conventionally constructed of a dielectric material such as a glass and is provided at the ends with electrodes which are connected with sources of electrical potential.
A variety of techniques have been proposed for providing the electrodes. In one approach the ends of the tubing are painted with a metal such as platinum. Platinum adheres poorly to the glass, however, and because it is soft it wears poorly. To improve the wear properties, the platinum may be overcoated with nickel, but layers of nickel that are sufficiently thick to resist peeling can fracture or flake at higher operating temperatures, owing at least in part to differential thermal expansion.
Generally, gas entering the upstream end of the conduit is in a turbulent flow condition for some distance, resulting in collisions of ions onto the lumenal surface. Because the conduit is constructed of a dielectric material, it is unable to carry away the electrical charge near the end of the conduit, and an undesirable charging effect results. Apparently, once laminar flow conditions are reached in the bore of the conduit, further downstream, ion collisions with the lumenal surface are substantially diminished, and charging effects are reduced. We have discovered that end-charging within the bore of the conduit can be reduced by coating the lumenal surface of an end portion of the tube with an electrically conductive material that carries away electrical charge resulting from ion collisions with the lumenal surface.
Accordingly, in one general aspect the invention features a conduit for conducting ions from a high pressure ion source to a mass analyzer in mass spectrometry apparatus, constructed of a dielectric material and having an electrically conductive coating on an end portion of the lumenal surface. In some embodiments the coating extends axially into the bore to the point at which, when the apparatus is in use, the flow of gas within the bore becomes laminar.
The end portions of the conduit are provided on at least the exterior surface with an electrically-conductive material serving as a contact for connection of the ends of the conduit to sources of electrical potential. Conveniently, the coating on the lumenal surface can be in electrical contact with the exterior contact.
In some embodiments a portion of the lumenal surfaces in at least one end of the conduit is coated with the electrically conductive material and, in some embodiments the two ends are similarly treated so that the conduit may be installed in the mass spectroscopy apparatus with either end oriented upstream. Between the end portions the conduit is nonconductive, for example by having no electrically-conductive material applied to the exterior and lumenal surfaces, over a length sufficient to permit the maintenance of the desired end-to-end potential.
In another general aspect the invention features a conduit for conducting ions from a high pressure ion source to a mass analyzer in mass spectrometry apparatus, which includes a tube constructed of a dielectric material and defining a capillary bore extending from end to end and having, affixed to at least one end of the capillary tube, an endpiece defining a bore having an electrically conductive lumenal surface and contiguous with the lumenal surface of the capillary tube at that end. Conveniently, the endpiece may also have an electrically conductive outer surface so that the outer surface of the endpiece provides an exterior contact for connection of the ends of the conduit to a source of electrical potential. In some embodiments the entire endpiece is constructed of an electrically conductive material, which may additionally be coated to provide good wear characteristics as well as electrical conductivity. Also conveniently, the endpiece can be configured as a short tube having dimensions similar to those of the dielectric tube. There may be an endpiece affixed to both ends of the dielectric tube and, conveniently, they may be similarly constructed so that the conduit may be installed with either end oriented upstream.
In some embodiments the desired end-to-end potential is in the range 500 V to 8 kV, or in some embodiments in the range 500 V to 5 kV. Usually the resistivity of the nonconducting portion of the conduit will be sufficiently high so that the current flow from end to end does not impracticably drain the power supply. In the interest of reducing current drain from the power source for the end-to-end potential, the nonconducting portion of the conduit will be longer where the conduit is made from a less poorly conductive dielectric, and can be shorter for less conductive dielectrics. For example, where a power supply is capable of delivering a maximum current of 1 mA at 5 kV, then the resistivity of the nonconducting portion of the conduit must be at least 5 Mxcexa9. Power supplies in conventional use for this purpose typically furnish 1 mA or less, and for use with such power supplies the nonconducting portion of the conduit should have a resistivity at least 10 Mxcexa9. In some embodiments the resistivity of the nonconductive portion of the conduit is less than 10 Mxcexa9 per cm, usually within the range 1 Mxcexa9 and 10 Mxcexa9 per cm. Depending upon the length and the current drain specification, the overall resistivity is usually in the range 10 Mxcexa9-100 Mxcexa9. Some glass materials, for example, require a conduit length about 1 cm/kV between electrically conductive end portions, so that the length of the nonconductive portion of the conduit should be as great as about 8 cm to maintain an end-to-end potential difference of 8 kV, for example. Such end-to-end potentials may be held over somewhat shorter lengths where a better dielectric material, such as a quartz or a ceramic, is used.
In some embodiments the coated portion within the bore extends from the end to a distance at least five times the bore diameter, more usually at least ten times the bore diameter.
In some embodiments the film or coating on the lumenal surface is generally thicker at the end, and thinner extending from the end within the lumen; and in some embodiments the coating provides more thorough continuity near the end than farther inward. In such embodiments the coating thickness can be said to taper within the lumen from a finite thickness near the end to substantially no thickness at some point inward, or the coating can be said to be attenuated inwardly progressively to substantially no thickness. The result of the taper or attenuation of the coating is a progressive reduction of conductivity, so that the conductivity of the coating or film at its innermost limit approaches that of the lumenal surface of the dielectric wall material.
In some embodiments the dielectric material of which the conduit is constructed is a glass or a quartz or a ceramic or a plastic such as a polytetrafluoroethylene (xe2x80x9cPTFExe2x80x9d, Teflon(copyright)) or a polyimid (Vespel(copyright)).
In some embodiments the electrically conductive material is a relatively nonreactive electrically conductive metal such as, for example, chromium or silver or gold or platinum. In some embodiments an additional electrically conductive coating is applied onto the surface of a portion of the electrically conductive coating and in conductive relation to it, usually onto the exterior portion, to provide mechanical and other properties not provided by the first-applied electrically conductive material.
In another general aspect the invention features a method for making an end-coated conduit for conducting ions from a high pressure ion source to a mass analyzer in mass spectrometry apparatus, by providing a tube made of a dielectric material and having suitable dimensions with an electrically conductive material, and applying a coating onto a portion of the exterior surface and onto a portion of the lumenal surface of an end of the tube.
In some embodiments the coating is applied by conventional sputter coating or vapor coating. The end of the conduit is presented to the source of coating material during the coating process in such a way that some coating material is directed into the opening of the bore of the conduit at the end and is deposited onto the lumenal surface there. In some embodiments the coating is applied by electrodeless plating.
In some embodiments the coating is applied by conventional chemical deposition techniques, using for example a ceramic paint or a metal paint such as a gold paint or silver paint, or, for example, a chrome hexacarbonate.
In another general aspect the invention features an end-coated conduit for providing an interface for conducting ions from a high-pressure ion source to a mass analyzer in mass spectrometry apparatus, made by the method of the invention.